Aerial spraying of insecticide began Thursday in the one-mile-square area of Miami where mosquitoes have infected people with the Zika virus, and officials reported some glimmers of progress.
“We are very encouraged by the initial results, which showed a large proportion of the mosquitoes killed,” Dr. Thomas R. Frieden, the director of the Centers for Disease Control and Prevention, said at a news conference here. But Dr. Frieden added, “This is going to take an intense effort.”
On Monday, faced with 14 locally transmitted infections, the C.D.C. took the unprecedented step of advising people to stay away from a location in the continental United States, urging pregnant women not to travel to the one-square-mile area where 12 of the cases are linked.
